As a result of the shrinking sizes of integrated circuits, photolithographic masks have to project smaller and smaller structures onto a photosensitive layer i.e. a photoresist dispensed on a wafer. In order to enable the decrease of the critical dimension (CD) of the structure elements forming the integrated circuits (ICs), the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex.
In the future, photolithographic masks will use even smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum. The term EUV mask denotes in the following a photolithographic mask for the EUV wavelength range (preferably 10 nm to 15 nm).
The optical elements for the EUV wavelength range will preferably be reflective optical elements. For the fabrication of an EUV optical element a multilayer structure or a multilayer film is deposited on a substrate having an ultralow thermal expansion (ULE). Fused silica is an example of a substrate used for EUV optical elements. The multilayer system typically comprises 80 to 120 alternating layers of molybdenum (Mo) and silicon (Si). A pair of a Mo—Si layer or a Mo—Si bilayer has a depth of approximately 7 nm. At the boundary of the Mo—Si layers a portion of the incident EUV radiation is reflected, so that a Mo—Si multilayer layer system ideally reflects more than 70% of the incident EUV radiation.
In addition to the multilayer structure, an EUV mask comprises a pattern or an absorbing pattern structure on top of the multilayer. For example, the EUV radiation absorbing pattern can be formed of titanium nitride, tantalum nitride, or chromium. The interaction of the EUV radiation absorbing portions and EUV radiation reflecting portions of the EUV mask generates in case of an illumination with EUV radiation the pattern to be presented in the photoresist dispensed on a semiconductor wafer.
Highest precision is required at the fabrication of EUV optical elements, in particular for EUV masks. Errors in the order of 1 nm can already cause errors in the image of the pattern structure on the wafer. Mask errors or defects which are apparent on the pattern of the wafer generated by the mask are called printing errors. Defects of different types can occur at various positions of the EUV mask leading to various effects.
EUV inspection and review systems operating at the illumination wavelength are already known. The U.S. Pat. Nos. 6,954,266, or 5,808,312 describe EUV inspection and review systems or tools operating in combination with a mask repairing system. EUV review systems are also denoted as EUV mask inspection microscopes (EUVM).
Further investigation methods for EUV masks are also known. The US 2009/0 286 166 discloses the use of an atomic force microscope (AFM) for the localization of concave defects on EUV masks. The U.S. Pat. No. 6,844,272 describes the determination of the height of the surface of EUV masks by the use of an interferometer.
Defects of the absorbing pattern structure may occur if absorbing material is missing at positions which should be opaque, or when absorbing material is existent at positions which should be dear. Further, dirt particles may be attached to the surfaces of EUV optical elements. This type of error results predominantly in amplitude errors. It can be recognized by a surface analysis of the EUV optical element; for example by using a scanning electron microscope (SEM). Using a known mask repairing system, as for example the MeRiT® system of Carl Zeiss SMS excessive material can be removed. An electron beam in combination with a suitable etching gas can be used for this task. Missing absorber material can for example be added by locally depositing chromium with the aid of an electron beam together with a respective precursor gas.
EUV optical elements can be washed or polished in order to dean their surfaces from disturbing particles or substances.
On the other hand, so-called buried defects can occur in EUV optical elements, i.e. EUV mirrors and/or EUV masks. In the following, the term “buried defect” means a defect or an error which is located on the substrate and/or within the multilayer structure of the EUV optical element. Buried defects lead to both, amplitude and phase errors, i.e. buried defects comprise amplitude and phase error portions. Buried defects are also called topological errors.
The U.S. Pat. No. 6,016,357 describes a method for correcting errors of the absorbing pattern structure by the measurement of focus stacks in phase shift masks (PSM) illuminated with deep ultraviolet (DUV) radiation. Further, this document describes a repairing method for removing excessive absorbing materials and for depositing missing absorbing material. This repairing method is denoted as compensational repair.
The U.S. Pat. No. 6,235,434 describes the repair of amplitude and phase errors of EUV masks. Independent of the type of error, the repair is done by compensation, i.e. correcting of the absorbing pattern by removing excessive material from the absorbing pattern structure or depositing absorbing material to the absorbing pattern structure, respectively. Further, the US 2005/0 157 384 also discloses the removal of material, whereas the U.S. Pat. No. 6,849,859 describes the adjustment of a thickness of a layer by depositing an additionally layer and by adjusting the thickness of the additional layer.
When excessive absorbing material is removed by an ion beam, ions are implanted in a buffer layer arranged between the multilayer film and the absorbing pattern structure. The implanted ions may vary the reflectivity of the corrected EUV mask portion. The JP 2008 085 223 A discloses a method to correct the reflectivity change induced by the implanted ions by respectively correcting the absorbing pattern structure.
The article “Study of critical dimensions of printable phase defects using an extreme ultraviolet microscope” by Y. Kamaji et al., Jpn. J. of Appl. Phys. 48 (2009), pp. 06FA07-1-06FA07-4 explains why pits are more often defects in multilayer films of EUV masks than bumps. Further, the article reports on the fabrication of programmed phase defects and their analysis in order to determine the resolution limit of an EUV microscope (EUVM).
The thesis of C. H. Clifford: “Simulation and compensation methods for EUV lithography masks with buried defects”, Electrical Engineering and Computer Sciences, University of California at Berkeley, Techn. Report No. UCB/EECS-2010-62 describes simulation methods which allow generating simulation configurations based on aerial images of defects. This document also reports on two methods for defect compensation by adjusting the absorber pattern of EUV masks.
The article “Natural EUV mask blank defects: evidence, timely detection, analysis and outlook” by D. van den Heuvel et al., SPIE/BACUS Conf. Proc. 2010, describes a method to combine aerial images, marks and AFM measurement data in order to localise and to measure an EUV defect which cannot be recognized in a scanning electron microscope (SEM). Moreover, this paper describes that both, pits as shallow as 3 nm and bumps just 3 nm high at the surface can results in critical printing defects buried in the multilayer.
The above mentioned documents do often not clearly distinguish between amplitude errors and phase errors of buried defects, i.e. of defects on substrates and/or multilayer films of EUV masks. The repair methods denoted as “compensational repair” addresses the amplitude error portions of buried defects, but ignores their phase error portions. FIG. 1 schematically illustrates the compensational repair of a multilayer defect by removing a portion of the absorbing pattern elements adjacent to the defect in order to compensate for the reduced reflectivity of the buried multilayer defect.
The compensational repair has the drawback that it results in a diminution of the process window at the wafer illumination, since an EUV mask compensated with this approach has a focus dependency which deviates significantly from the ideal focus characteristics. Moreover, the compensational repair method cannot be applied to correct defects in EUV mirrors not having an absorbing pattern structure.
It is therefore one object of the present invention to provide a method and an apparatus for analysing and/or repairing of a defect of an EUV mask, which at least partially overcome the above mentioned drawbacks of the prior art.